The present invention relates to a reflective guest-host liquid-crystal display device. More particularly, the present invention relates to technology for improving the efficiency of utilizing incident light by removing a polarizer.
Liquid-crystal display devices operate in various modes. At the present time, the dominant mode is a TN (Twisted Nematic) or STN (Super Twisted Nematic) mode in which a twist-oriented or supertwist-oriented nematic liquid crystal is used. However, in respect of operating principles, these modes require a pair of polarizers. Since light absorption occurs in the polarizers, transmittance is low, and a bright display screen cannot be obtained. In addition to these modes, a guest-host mode which uses a dichroic dye has been developed. A liquid-crystal display device of a guest-host mode makes a display by using the anisotropic properties of the absorption coefficient of the dichroic dye added to the liquid crystal. When the dichroic dye of a bar-shaped structure is used, the dye molecules have the property of being oriented parallel to the liquid crystal molecules. Therefore, when the molecule orientation of the liquid crystal is changed by applying an electric field, the orientation of the dye is also changed. Since whether or not the dye develops a color depends upon the orientation, it is possible to switch the coloring and colorless states of the liquid-crystal display device by applying a voltage.
FIGS. 7A and 7B show the structure of a Heilmeier-type guest-host liquid-crystal display device. FIG. 7A shows a state in which no voltage is applied, and FIG. 7B shows a state in which a voltage is applied. This liquid-crystal display device uses a p-type dye and a nematic liquid crystal (Np liquid crystal) whose dielectric anisotropy is positive. The p-type dichroic dye has an absorption axis which is substantially parallel to the molecular axis, and strongly absorbs polarized components Lx parallel to the molecular axis and hardly absorbs polarized components Ly perpendicular to the polarized components Lx. In the state shown in FIG. 7A in which no voltage is applied, the polarized components Lx contained in the incident light are strongly absorbed by the p-type dye, and the liquid-crystal display device is made colored. As compared with this, in the state shown in FIG. 7B in which a voltage is applied, the Np liquid crystal having positive dielectric anisotropy is turned on in response to the electric field, and in accordance with this, the p-type dye is also oriented in a perpendicular direction. For this reason, the polarized components Lx are hardly absorbed, and the liquid-crystal display device is made colorless. The other polarized components Ly which are contained in the incident light are not absorbed by the dichroic dye regardless of the voltage applied state or the voltage non-applied state. Therefore, in the Heilmeier-type guest-host liquid-crystal display device, a single polarizer is interposed beforehand to remove the other polarized components Ly.
In the guest-host liquid-crystal display device using a nematic liquid crystal, a dichroic dye which is added as a guest is oriented in the same way as the nematic liquid crystal. Although the polarized components parallel to the orientation of the liquid crystal are absorbed, the polarized components perpendicular to said first polarized components are not absorbed. Therefore, in order to obtain a sufficient contrast, a polarizer is disposed on the incident side of the liquid-crystal display device, and the polarization direction of the incident light is aligned with the orientation of the liquid crystal. However, when this is done, since 50% (actually 40% or so) of the incident light is lost in principle by the polarizer, the display becomes dark as in the TN mode. As a method of reducing this problem, the mere removal of the polarizer causes the on/off ratio of absorbance to be decreased considerably. Therefore, this is not appropriate, and various improvement measures have been proposed. For example, a construction has been proposed in which a polarizer is removed from the incident side, while a xcex/4 phase shifter and a reflection plate are mounted on the emergence side. In this method, the polarization directions of two polarized components located perpendicularly to each other are rotated by 90xc2x0 in the forward path and the backward path, and thus the polarized components are interchanged with each other. Therefore, in the off state (the absorption state), each polarized component will be absorbed in either the incident optical path or the reflection optical path. However, since a xcex/4 phase shifter and a reflection plate are provided externally in this structure, the liquid-crystal display device itself must be made to be a transmission type. In particular, in a case in which an active matrix type structure is adopted to achieve a high resolution and to display a moving image, since thin-film transistors for driving pixel electrodes are integrated on a substrate, the pixel aperture ratio is low in the transmission type, and a considerable portion of the incident light is shut out. Therefore, even if the polarizer is removed, the screen of the display device cannot be made to be markedly bright. Meanwhile, as another measure, by making it possible to absorb all of the polarized components contained in the incident light by using a cholesteric liquid crystal, the polarizer can be removed. However, this method has a drawback in that an halftone display cannot be made due to the strong hysteresis of the cholesteric liquid crystal.
A phase shifter will now be described below.
Generally speaking, a phase shifter refers to a birefringence plate (crystal plate) for providing a predetermined optical path difference (therefore a phase difference) between linearly polarized light vibrating at mutually perpendicular directions when said light passes through a plate. When the thickness of the birefringence plate is denoted as d, the refractive index of the linearly polarized light which vibrates along the electrical principal axes perpendicular to each other as n1 and n2, the optical path difference is given as |n1xe2x88x92n2|. Phase shifters having optical path differences of xcex/4, xcex/2 and xcex/1 (xcex is the wavelength of the light used in a vacuum) are called a xcex/4 phase shifter, a xcex/2 phase shifter, and a xcex/1 phase shifter, respectively, and correspond to phase shifters of xcfx80/2, xcfx80, and 2xcfx80, respectively. For example, the xcex/4 phase shifter is a birefringence plate whose thickness is so determined as to introduce an optical path difference of a 1/4 wavelength between the linearly polarized light which vibrates perpendicular to each other. A thin film or the like in which a muscovite is cleaved to a proper thickness is used. Or, a synthetic resin plate or the like in which molecules are oriented in one direction is used. When linearly polarized light having a direction of 45xc2x0 with respect to the principal axis is made to enter this plate, the transmitted light becomes circularly polarized light.
The xcex/4 phase shifter has various uses, and in recent years, it is used in a polarization control element of a flat-panel-type display, such as a liquid-crystal display device. However, for a conventional xcex/4 phase shifter, a thin film in which a muscovite is cleaved to a proper thickness, a synthetic resin plate in which molecules are oriented in one direction, or others is used. It is difficult to prepare such a xcex/4 phase shifter of a large area, and the xcex/4 phase shifter cannot be incorporated into a large flat panel display. Further, since the conventional xcex/4 phase shifter has a considerable degree of thickness, it cannot be incorporated into the inside of a liquid-crystal cell or the like which forms a flat panel display, and serious structural limitations occur.
In order to solve the above-described problems of the prior art, a reflective guest-host liquid-crystal display device having the construction described below is proposed. That is, the basic components of the reflective guest-host liquid-crystal display device of the present invention include a first substrate for receiving incident light, on which substrate transparent electrodes are formed; a second substrate which has a reflective film formed thereon and which is positioned so as to face the first substrate with a predetermined spacing; and an electro-optical element which is held in said spacing and which performs optical modulation. The electro-optical element includes a lamination structure containing a guest-host type liquid-crystal layer which contains a dichroic dye and which is uniformly oriented with respect to the transparent electrode, and an optical thin-film layer which has a predetermined optical anisotropic axis and which is formed along the reflective film. The liquid-crystal layer changes between an absorption state and a transmission state in response to an applied voltage. In the absorption state, the liquid-crystal layer substantially absorbs first vibration components contained in the incident light, and is substantially permeable to second vibration components perpendicular to the first vibration components. In the transmission state, the liquid-crystal layer is substantially permeable to both vibration components. The optical thin-film layer is interposed in the forward and backward paths of the second vibration components reflected by the reflection electrode, the second vibration components are converted into the first vibration components and these components reenter the liquid-crystal layer in the absorption state. As a feature, in the electro-optical element, the optical thin-film layer is formed from a polymer liquid-crystal material containing liquid-crystal molecules which are uniaxially oriented along the optical anisotropic axis, and functions as a xcex/4 phase shifter. In addition, the electro-optical element contains a passivation layer which is interposed between the optical thin-film layer and the liquid-crystal layer, which passivation layer physically separates the optical thin-film layer and the liquid-crystal layer from each other so as to protect the polymer liquid-crystal material.
Preferably, the passivation layer has the function of causing the liquid-crystal layer to be homeotropically oriented or homogeneously oriented. Further, preferably, the passivation layer is formed from a photosensitive material, and can be formed into a pattern by an exposure and development process. Still further, preferably, the optical thin-film layer is formed into a pattern as well with the passivation layer which is formed into a pattern as a mask. Yet still further, preferably, the optical thin-film layer contains coloring areas which are divided into the three primary colors, and a pattern is formed for each coloring area and color filters are formed.
The vibration components along the orientation direction of the liquid-crystal layer in the absorption state are absorbed by a dichroic dye which is oriented in the same direction. However, since the vibration components perpendicular to this intersect the orientation of the dye molecules, they are hardly absorbed. In other words, the vibration components hardly receive light modulation. However, according to the present invention, these vibration components pass through the liquid-crystal layer, after which they enter the optical thin-film layer. Further, after the vibration components are reflected by the reflection electrode, they pass through the optical thin-film layer again. Therefore, it follows that these vibration components have passed two times through the optical thin-film layer which functions as a xcex/4 phase shifter, and their vibration direction (the polarization direction) is rotated 90xc2x0. In doing so, since the direction coincides with the orientation direction of the liquid crystal in the absorption state, these vibration components are absorbed. In this way, since all the vibration components contained in the incident light are always absorbed in either the forward path or the backward path, an external polarizer is not required. Therefore, even if the polarizer is removed, a contrast which is substantially comparable to that of a transmission-type guest-host liquid-crystal display device with a polarizer can be obtained.
In the meantime, in the present invention, a passivation layer is interposed between the optical thin-film layer and the liquid-crystal layer which constitute the above-described electro-optical element. This passivation layer physically separates the optical thin-film layer and the liquid-crystal layer from each other so as to protect the polymer liquid-crystal material. Since the optical thin-film layer formed from a polymer liquid-crystal material is separated from the guest-host type liquid-crystal layer by this passivation layer, a liquid-crystal display device which is free from mutual dissolution and having high reliability can be obtained. If this passivation layer were not interposed, the optical thin-film layer comes into direct contact with the liquid-crystal layer, and the polymer liquid-crystal material from which the optical thin-film layer is formed might be dissolved into the guest-host type liquid-crystal layer. Further, when a selection of a material for this passivation layer is appropriately made and the passivation layer is subjected to a process, such as rubbing, it is possible for the passivation layer to function as an alignment film. That is, the guest-host type liquid-crystal layer in contact with the passivation layer can be homeotropically or homogeneously oriented. There is no need to separately provide an alignment film, there is no increase in the number of steps, and the manufacturing costs can be reduced. In addition, since a photosensitive material is used as a passivation layer, patterning is possible by an exposure and development process. By using a patterned passivation layer as a mask, color filters can be formed on a substrate, making it extremely easy to cause a reflective guest-host liquid-crystal display device to make a color display. To be specific, by using the patterned passivation layer as a mask, the optical thin-film layer can be divided into coloring areas which are separated into the three primary colors, and these areas can be used as color filters.
A method of depositing a polymer liquid crystal which forms an optical thin-film layer in accordance with the present invention comprises the following steps. Initially, an orientation step is performed to orient the surface of a transparent substrate along a predetermined orientation direction. Next, a film deposition step is performed in which a polymer liquid-crystal material capable of making a phase transition between a nematic liquid-crystal phase on the high temperature side and a glass-solid phase on a low temperature side with a predetermined transition point as a boundary is deposited to a predetermined film thickness on the substrate. Finally, a heat-treating step is performed in which after the substrate is heated temporarily above the transition point, it is slowly cooled down to the room temperature below the transition point, and a uniaxial optical thin film is formed by orienting the deposited polymer liquid-crystal material along the orientation direction. By controlling the film thickness of the uniaxial optical thin film, it is possible to provide the function of the xcex/4 phase shifter.
Preferably, in the orientation step, an alignment film of a polyimide film or the like is deposited on the surface of the substrate, after which the polyimide film is rubbed along the orientation direction. Still preferably, in the film deposition step, a polymer liquid-crystal material is deposited by spin coating, dipping, or printing. Yet still preferably, in the heat-treating step, a polymer liquid-crystal material having a transition point of above 100xc2x0 C. and having liquid-crystal molecules introduced into the main chain or the side chain of the polymer is heated or slowly cooled.
The xcex/4 phase shifter obtained in this way can be easily formed into a large area, and can be incorporated into a large flat panel display. Further, since the xcex/4 phase shifter has a thin-film structure, it is possible to easily incorporate it into the inside of a liquid-crystal cell or the like which forms a flat panel display.
The above and further objects, aspects and novel features of the invention will become more apparent from the following detailed description when read in connection with the accompanying drawings.